Evil mode overwrites the mouse-2 event which is used by Agda to go to the definition of clicked identifier.
To restore the desired behavioru, just add the following line after enabling Evil (require 'evil)
(define-key evil-normal-state-map [mouse-2] 'agda2-goto-definition-mouse)
Thank you a ton! I tried
global-unset-key
but it couldn't work ;(